This invention relates to heat transfer apparatus and more particularly to counterflow type heat exchangers for use in systems which provide extremely low temperatures for cryogenic devices and the like.
In the construction of a perforated plate heat exchanger of high effectiveness, it is essential to make the plates of a material that has high thermal conductivity, and the spacers between the plates of a material that has low thermal conductivity. A simple configuration for a perforated plate heat exchanger is a cylindrical stack of plates having a central axial passage separated from an annular passage by spacer rings between the plates. Such a heat exchanger is disclosed, for example, in U.S. Pat. No. 3,228,460, which is incorporated herein by reference.
In a typical embodiment, the plates are made of copper and the spacer rings are cut from sheet plastic. The spacer rings are often bonded to the plates by an epoxy adhesive. It has been found, however, that if the heat exchanger is used in a closed-cycle cryogenic system, the presence of organic substances such as plastic presents a problem, because over time, the working fluid will become contaminated by gases evolved from the plastic. These gases will freeze out on the coldest parts of the system and degrade performance. Therefore, a heat exchanger best suited for use in cryogenic systems will contain no organic materials.